Positive Outgoing Longwave Radiation Anomalies Research Articles

Role of Cross-Equatorial Waves in Maintaining Long Periods of Low Convective Activity over Southern Africa

Abstract Periods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropical–extratropical cloud bands from the southern African landmass into the southwestern Indian Ocean. The wave train is also seen to generate an anticyclonic anomaly over southern Africa, resulting in suppressed convective activity. Understanding the causes of the long periods of low convective activity will help in improving their predictability and also the predictability of seasonal rainfall over southern Africa.

Role of Rossby Waves in the Remote Effects of the North Indian Ocean Tropical Disturbances

Abstract Remote effects due to the tropical disturbances in the north Indian Ocean are investigated by analyzing long-lasting (≥5 days) tropical disturbances, which reached at least the strength of tropical storms. The present analysis is carried out for both the pre- and postmonsoon periods. The spatial and temporal distribution of the outgoing longwave radiation (OLR) during the premonsoon disturbances over the Bay of Bengal reveals several interesting features. Temporal distribution of the OLR anomalies shows that the intraseasonal oscillations play an important role in the formation of those disturbances. The spatial distribution of the OLR anomalies shows a dipole with negative OLR anomalies over the bay and positive OLR anomalies over the Indonesian region. The atmospheric response to the negative OLR anomalies results in positive temperature anomalies over northwest India, Pakistan, Afghanistan, Iran, and Saudi Arabia, remote from the disturbance; and the response to the positive anomalies causes slight increase in the sea surface temperature of the Arabian Sea. Negative OLR anomalies are also seen over western Japan due to the Rossby waves generated by the heating over the Bay of Bengal besides the enhancement of the so-called “Pacific–Japan” teleconnection pattern. However, the analysis shows that the postmonsoon disturbances over the Bay of Bengal and the disturbances formed over the Arabian Sea in both pre- and postmonsoon seasons do not develop remote teleconnections associated with the above type of Rossby wave mechanism. These results are significant for the short- to medium-range weather forecast over a wide range covering Japan, Pakistan, Afghanistan, Iran, and Saudi Arabia.

Understanding of anomalous Indian Summer Monsoon rainfall of 2002 and 1994

This study focuses on understanding of the deficient rainfall of 2002 and excess rainfall of 1994 of the Indian Summer Monsoon and its association with large-scale circulation features. Outgoing longwave radiation (OLR) and 200 hPa flow regimes have been compared during a deficient year (2002) with those during an excess rain year (1994) for the northern summer months. The main emphasis is on the month of July (2002) when rainfall was the lowest during the past 100 years. Positive OLR anomalies of substantial magnitude are prominent over large parts of the Indian Ocean, Arabian Sea, most parts of India. Negative OLR anomalies prevailed over the north Pacific in July 2002. These positive OLR anomalies during the deficient Summer Monsoon of 2002 indicate very weak convection over the Indian subcontinent. The opposite patterns of OLR anomalies are found in the excess rain monsoon of 1994. Examination of upper tropospheric (200 hPa) circulation fields indicates significant changes in velocity potential, stream function, and geopotential height anomalies during 2002 and 1994. The field of geopotential height anomalies shows cyclonic (anticyclonic) anomalous circulation developed over the Caspian Sea near central Asia during 2002 (1994) rain years. The upper level stream function anomalies show weak (strong) upper level high in 2002 (1994) over large parts of India and surrounding areas in July.

Links between rainfall variability on intraseasonal and interannual scales over western Tanzania and regional circulation and SST patterns

This study investigates the circulation anomalies associated with the intraseasonal evolution of wet and dry years over western Tanzania (29–37° E, 11.5–4.75° S) and how the onset and withdrawal of the rainy season as well as its wet spell characteristics are modified. It is found that for wet years, the rains begin earlier and end later, with strong wet spells occurring during the season, and there tend to be a greater number of moderate wet spells (although not necessarily more intense wet spells) than in dry years. In dry years, late onset and early cessation of the rainy season occur, often with an extended dry spell soon after the onset, and there tend to be a greater number of dry spells within the season. Large negative outgoing long wave radiation (OLR) anomaly values tend to be located between 20° and 40° E with anomalous westerly flow at 850 hPa occurring across the continent from 10° E to the tropical western Indian Ocean during wet spells in the anomalously wet seasons. Anomalously dry seasons are characterised by large positive OLR anomalies over 30–50° E as well as easterly anomalies at 850 hPa and westerly anomalies at 200 hPa. Eastward propagating intraseasonal anomalies are slower during the wet years implying that the convection remains over Tanzania longer. On the intraseasonal scale, Hovmoeller analyses of OLR and 850 and 200 hPa zonal wind indicate that convection over western Tanzania may be associated with a flux of moisture from the tropical southeast Atlantic and Congo basin followed by weak easterlies from the tropical western Indian Ocean.

The Teleconnection between the Western Indian and the Western Pacific Oceans

Abstract Shown in this study are a teleconnection pattern relating outgoing longwave radiation (OLR) anomalies over the western Pacific Ocean and sea surface temperature anomalies (SSTAs) over the western Indian Ocean over two seasons [September–October–November (SON) and December–January–February (DJF)] at zero lag from observations and atmospheric general circulation model (AGCM) integrations. This teleconnection pattern suggests that a positive SSTA in the SON and DJF seasons over the western Indian Ocean increases the contemporaneous positive OLR anomalies over the western Pacific Ocean. This teleconnection pattern is also simulated by the Center for Ocean–Land–Atmosphere studies (COLA) AGCM forced with observed SST. From the experimental COLA AGCM runs (wherein the Pacific Ocean SST variability is suppressed except for the climatological annual cycle), it is diagnosed that the interannual variability of OLR over the western Pacific Ocean persists because of this teleconnection. This teleconnection pa...

The South Atlantic Convergence Zone: Intensity, Form, Persistence, and Relationships with Intraseasonal to Interannual Activity and Extreme Rainfall

The characteristics of intensity, geographical location, and persistence of the South Atlantic convergence zone (SACZ) during the austral summer are investigated. Intensity and spatial features of the SACZ are identified by performing a factor analysis of structural properties of outgoing longwave radiation (OLR) data. The first two leading factors explain 65% of the total variance of structural properties and characterize the SACZ according to intensity and location (oceanic versus continental). An index is constructed based on the magnitude of the factor scores to identify intense (weak) and oceanic (continental) SACZ. The intense SACZ category is associated with negative OLR anomalies over a large area of tropical South America, extending from the western Amazon to the Atlantic Ocean. The weak SACZ category is observed with positive OLR anomalies over tropical South America and negative OLR anomalies over southeastern South America. Oceanic and continental aspects of the SACZ are related to a midlatitude wave train pattern. The Madden–Julian oscillation (MJO) modulates intense SACZ events with persistence longer than 3 days. Interannual variability of persistent events indicates that the ratio of oceanic to continental SACZ as well as their frequency depends on the phase of El Niño–Southern Oscillation (ENSO). Occurrence of extreme rainfall in Brazil is discussed in the context of variations in the SACZ and MJO. Intense (weak) SACZ increases (decreases) the 95th daily rainfall percentile over central-eastern Brazil compared to the climatology. Oceanic (continental) SACZ increases (decreases) the 95th daily rainfall percentile over southeastern Brazil. The MJO phase characterized by suppression of convective activity over Indonesia and enhancement over the central Pacific increases the 95th daily rainfall percentile over north-northeastern Brazil, whereas opposite features are observed for the phase of the MJO characterized by the enhancement of convection over Indonesia and suppression over the central Pacific.

The Role of the Local Hadley Circulation over the Western Pacific on the Zonally Asymmetric Anomalies over the Indian Ocean

The onset processes of the zonally asymmetric anomalies of convection and sea surface temperature (SST) over the tropical Indian Ocean are investigated with considering seasonal evolution, and interannual variability, of the large-scale convection anomalies in the Asian summer monsoon, using outgoing longwave radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern of the convection anomalies is particularly dominated in boreal autumn. Some recent studies have noted that these anomalies, based on the atmosphere-ocean coupling phenomenon, can be developed and maintained by itself. The time evolution shows that the eastern part of the zonally asymmetric anomalies over the Indian Ocean lead the western part of those. In July, the negative SST anomalies and positive OLR anomalies first appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatological southeasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SST cooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetric anomalies in the following autumn. It is suggested that this southeasterly wind acceleration over the southeastern Indian Ocean is closely linked to the meridionally asymmetric anomalies of convection, between the maritime continent and the South China Sea/Philippine Sea (SCS/PS). That is, the intensification of the local Hadley circulation over the western Pacific associated with the enhanced convection over the SCS/PS, and suppressed convection over the maritime continent, is found to be a clear precursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been noted that the convection anomalies over the southern and northern parts of the meridionally asymmetric anomalies over the western Pacific are not always the opposite sign, and seem to have different interannual variability respectively. It is likely that the former might be strongly influenced by the ENSO, through the Walker circulation anomalies and the latter might be affected by the modulation of the intraseasonal variation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomalies is also suggested from the occurrence of the intensification of the local Hadley circulation in boreal summer.

The Influence of the Madden–Julian Oscillation on Ocean Surface Heat Fluxes and Sea Surface Temperature

The Madden‐Julian oscillation (MJO) involves pronounced variations in convection and large-scale circulation throughout the tropical troposphere. In addition, the MJO is also related to dynamic and thermodynamic variability near the surface and the upper ocean. This study uses observational data to characterize the changes in surface heat fluxes and sea surface temperature (SST) during the life cycle of the MJO. Variations in convective activity are described with outgoing longwave radiation (OLR) during the period January 1985 through September 1994. International Satellite Cloud Climatology Project data (January 1985‐ April 1991) and European Centre for Medium-Range Weather Forecasts surface analyses (January 1985‐December 1994) are used to derive surface fluxes of net shortwave radiation (SW), latent heat ( E), their difference (Q 5 SW 2 E), and SST. The spatial patterns of OLR, SW, E, Q, and SST anomalies reveal that the region of positive OLR anomalies that precede the occurrence of enhanced convection is associated with positive SW and negative E anomalies, which result in positive Q anomalies. The prevailing conditions in the region of positive Q anomalies favor the development of positive SST anomalies, which lead to variations of enhanced convection. In contrast the region of negative OLR anomalies is associated with negative SW and positive E anomalies. These conditions induce negative Q anomalies, which favor the formation of negative SST anomalies. The above results suggest a possible feedback between the oscillation and intraseasonal variations in SST and this may be an important mechanism for numerical simulations of the life cycle of the MJO.